Bank notes, checks, tickets, admission tickets and other documents of value, in particular made of paper, for the purpose of forgery-proofness are equipped with security features, with the help of which the authenticity of the documents of value is checkable.
In this context there has been proposed to apply markings to a document by means of laser radiation, so as to thereby achieve an irreversible and visually easily detectable alteration of the document. For example in DE 28 36 529 C2 it is proposed to burn out the serial number from an ink layer by means of an appropriately controlled laser beam. In EP 0 918 649 B1 it is proposed to repeat the identification number at a different place of the document by a local reduction of the document thickness by means of laser etching. In the two above-mentioned cases the material is removed by means of laser radiation.
In contrast to that other approaches to the problem provide, that the substrate material is merely blackened by means of laser radiation. As to achieve particularly well readable and sharp-edged markings, it is also known to admix absorbents and carbon forming agents to the paper, for example microground plastics (DE 197 32 860 A1).
DE 198 22 605 A1 takes a different path. Therein it is proposed that the paper substrate surface at first is treated with laser energy, so as to structurally alter the surface, and to subsequently provide this surface with an opaque coating, for example by printing, lacquering and/or metalizing. The prior alteration of the surface leads to a noticeable alteration of the ink density, the ink location, the luster and/or the reflection in the subsequently applied coating, the result of which is a visible security feature.
The above-mentioned security features all are visually noticeable security features. But there exists a basic need for further, new security features, in particular for security features, which are perceptible for another sense organ, for example detectable with the sense of touch, i.e. tactile security features.